How to make an automated dependency injector?

Question

So I'm developing a framework and up until now I've been using a container that can be called statically throughout the application like so:

$router = new Router;
Container::set('router', $router); // this can now be called anywhere

This then in any other file within the app, can be called like so:

$router = Container::get('router'); // get the router object with all current data
$router->add_route('/blog', '/Path/To/Controller/Blog@index');

Within the controller I would then have:

class Blog_Controller extends Base_Controller
{
    public function __construct()
    {
        $this->blog_model = Container::get('Blog_Model'); // this is instantiated OR if it has already, the container returns the existing object INCLUDING all of its properties thus far.
    }

    public function view($id)
    {
        $this->blog_model->get_post($id);
    }
}

Though, I've read up on how to create DI Containers that inject the dependencies when the controller is called like in the example below:

class Blog_Controller extends Base_Controller
{
    public function __construct(Blog_Model $blog_model)
    {
        $this->blog_model = $blog_model; // this is instantiated OR if it has already, the container returns the existing object INCLUDING all of its properties thus far.
    }

    public function view($id)
    {
        $this->blog_model->get_post($id);
    }
}

I'm having trouble figuring out how to effectively do so.

How would I get Blog_Model to be instantiated (or returned if it already has been) without having to manually do new Blog_Model.